Changes Involving Unbroken Chromosomes 



159 



figure 1 1-9. The chromosomal complement of a normal human female. 

 Cell was in mitotic metaphase {hence chromosomes appear double except at 

 the centromere) when squashed and photographed. (Courtesy of T. C. Hsu.) 



The frequency among live births of 

 Down's syndrome due to trisomy has been 

 determined as approximately 0.2%. Most 

 cases of Down's syndrome occur among the 

 children of older mothers and are due pri- 

 marily to nondisjunction during oogenesis. 

 If other chromosomes have a similar fre- 

 quency of nondisjunction, there might be a 

 minimum of 4.4% (22 X 0.2% ) of zygotes 

 autosomally trisomic at conception. There 

 might also be another 4.4% of zygotes that 

 are autosomal monosomies, due to the equal 

 chance that the haploid meiotic product 

 complementary to the one which is disomic 



— the nullosomic one — becomes the egg. In 

 fact, more nullosomic than disomic gametes 

 are expected, since a chromosome left out 

 of one daughter nucleus need not be included 

 in the sister nucleus. Supporting a normally 

 high frequency of aneusomy is the observa- 

 tion that about one quarter of aborted hu- 

 man fetuses show a chromosomal derange- 

 ment. It is expected, moreover, that main 

 conceptions involving aneusomy. especially 

 monosomy, are lost so early in pregnanes 

 that they go unnoticed. 



Nondisjunction leading to aneusomy can 

 also occur in the paternal germ line of man, 



